With the development of olefin polymerization techniques, much progress has been made on the catalysts used in the polymerization techniques. High efficiency catalysts, by virtue of their good polymerization property and mature application technologies, still play an important part in catalysts for polymerization of olefins. With more and more attention being paid to the development and production of new structural polyolefin resins, there are increasingly more requirements for comprehensive properties of olefin polymerization catalysts. Catalysts are required not only to adapt to processing apparatus, but also to be capable of adjusting and controlling structures of resins. Years of explorations and researchers have shown that Mg—Ti based high efficiency catalysts are suitable for such applications.
Currently, Mg—Ti based high efficiency catalysts are mainly prepared by means of dissolving and precipitating, i.e., dissolving a magnesium compound first in a solvent and then precipitating. For example, JPS54-40293 discloses dissolving of magnesium compounds with titanate; JPS56-811 and JPS58-83006 disclose dissolving of magnesium compounds with alcohols, aldehydes, amines, carboxylic acids; JPS58-19307 discloses dissolving of magnesium compounds with organic phosphorus compounds; and JPS58-183708 discloses dissolving of magnesium compounds with a mixture of organic epoxy compounds and organic phosphorus compounds (phosphate compounds).
While the above solutions for dissolving magnesium compounds can to some extent eliminate the defects of grinding method, they could still be improved. For example, in JPS5440293, JPS58-19307, and JPS58-183708, when the catalyst separated from the solution of the magnesium compound as a precipitate is used in olefin polymerization, its catalyst activity is relatively low and decays distinctly with continuation of the polymerization, and the bulk density of the obtained polymer is also low.
In addition, Mg—Ti based high efficiency catalysts are also prepared by chemical reaction methods, in which chemical materials such as organic magnesium metal compound, chlorinating agent, and transitional metal titanium compound are used to prepare different types of catalysts. Such methods are disclosed in Chinese patents CN1158136, CN1299375, CN1795213, and U.S. Pat. No. 3,787,384, U.S. Pat. No. 4,148,754, U.S. Pat. No. 4,173,547, U.S. Pat. No. 4,508,843, and U.S. Pat. No. 5,124,296. Though performance of such Mg—Ti catalysts can be easily modulated, catalysts obtained are not in a good particle form, and have a wide distribution and even multimodal distribution, which is not good for long-term steady operation of polymerization plants.
In view of the foregoing, it is desirable to provide an olefin polymerization catalyst component that is high in activity, slow in decay of activity, good in hydrogen response, and conducive to long-term steady operation of polymerization plants.